There has been considerable development in pneumatic presses because of the high quality of the juice that they are capable of obtaining, in particular from grape, most by virtue of the juice being extracted gently due to the low pressures of about 2 bars that are exerted.
However, they suffer from considerably drawbacks which have limited their spread. The main drawback of prior art pneumatic presses lies in their discontinuous operation. These prior art pneumatic presses have a fruit loading hatch which is also used for removing residue, in particular grape marc. The operating cycle of these prior art machines thus comprises a loading stage during which the drum is stationary, optionally preceded by a stage during which a vacuum is applied to its membrane, followed by a juice extraction stage during which the membranes are progressively inflated while the drum is rotated, and a residue unloading stage during which the drum is again at rest.
The loading and unloading stages occupy a relatively large fraction of the total cycle time and, above all, the fruit is stored prior to being brought to the press and this can give rise to rapid oxidizing of the fruit, particularly if the fruit has been harvested mechanically, and such oxidizing reduces the quality of the resulting juice.
Further, most prior art pneumatic presses are ill-adapted to selecting juice as a function of the pressure at which it was obtained. Such selection requires the machine to be stopped prior to increasing the pressure and changing the juice-collecting tank.
Proposals have also been made to use a non-perforated drum fitted with juice flow ducts for selecting juices more easily. However, such machines also operate discontinuously and, in addition, such flow ducts are difficult to provide.
The present invention seeks to mitigate these drawbacks of prior pneumatic presses by providing a novel press which is particularly well adapted to semi-continuous or permanent operation enabling fruit, and in particular grapes, to be loaded on a permanent basis and also making it easy to select juice as a function of the pressure at which it was extracted. In all embodiments of the invention, waste evacuation is facilitated by using a large evacuation opening.